sphinx: Update to 7.2.6, python 3.12, ubuntu-24.04

Update sphinx to 7.2.6, python 3.12. This requires to update
.readthedocs.yml to use ubuntu-24.04 [1], because keep in sync sphinx in
readthedocs with with the one for local development and github CI.
Raising python makes sense, as development is usually done on newer
distros, we were still using old python 3.6.

This fixes the problem on local development trying to run older sphinx
5.3 on python 3.13 requires to use imghdr, which was removed from
standard library (alternatively we'd need to install it from pip via
adding standard-imghdr into requirements.txt).

[1] https://about.readthedocs.com/blog/2024/06/ubuntu-24-04/

Fixes: b900b79 ("doc: Add sphinx to requirements.txt")
Signed-off-by: Petr Vorel <pvorel@suse.cz>

Author: pevik

.readthedocs.yml

@@ -1,9 +1,9 @@
 version: 2
 
 build:
-  os: "ubuntu-22.04"
+  os: "ubuntu-24.04"
   tools:
-    python: "3.6"
+    python: "3.12"
   apt_packages:
     - autoconf
     - enchant-2

doc/requirements.txt

@@ -1,6 +1,6 @@
 # Use the same sphinx as on readthedocs.org. When updated, make sure
 # sphinx-rtd-theme is compatible with sphinx.
-sphinx==5.3.0
+sphinx==7.2.6
 sphinx-rtd-theme==2.0.0
 
 linuxdoc==20231020